The present invention relates generally to the processing of carbon fibers from a carbonaceous pitch, and more particularly to the oxidation or thermosetting stage in the formation of said fibers and specifically to a spool used for holding the fibers during oxidation.
What is referred to as carbon fiber is produced by first spinning an anisotropic pitch (or fraction of such pitch) for producing fiber, oxidizing or thermosetting the "green" fibes, and thereafter carbonizing the oxidized fibers at a high temperature. See U.S. Pat. No. Re. 27,794 and U.S. Pat. No. 4,208,267 for descriptions of this process.
The oxidation or thermosetting of the green fibers is a critical step in this process because the green fiber undergoes specific known chemical changes which permit the development of high tensile strength in the finished carbon fiber. Moreover, an oxidized carbon fiber is easier to handle than an unoxidized green fiber. There is an increase in tensile strength. For example, an unoxidized fiber would have a tensile strength of approximately 5 ksi, whereas the same fiber after oxidation would have a tensile strength of approximately 15 ksi.
Presently, for oxidizing carbon fiber made from petroleum or coal-derived oil pitch, it is the practice to pass the as-spun green fiber through a heated zone using low tension or move it on a conveyor belt. For 10 to 15 micron fiber, an oxidation time of one hour is necessary due to the diffusion time of oxygen into the fiber. Oxidizing agents may be used to decrease this time and to improve the efficiency of the oxidation or thermosetting step. However, it would be advantageous if it were possible to oxidize effectively the fibers on a spool, since this would be a more economic oxidation as well as facilitating the process.
To oxidize efficiently a maximum length of carbon fiber it would be desirable to concentrate the fiber in an oxidizing environment. This may be done by winding the fiber on a spool or bobbin. The outer coils of wound fiber may block oxidation of the inner wound coils, and therefore the spool has to be designed to avoid this problem.
Oxidation on a spool obviates the need for the elaborate equipment needed to oxidize the green fiber obtained from the spinning stage. Also, due to the greater packing density, the length of time required to process the material is much smaller when the material is on a spool. For example, a one-pound spool of 1,000 filaments contains 8,635 feet of fiber. A presently used oxidation oven for unwound fiber would be at least 50 feet in length and the retention time of the fiber in that oven would be one hour or more. Therefore, it would require about 172 hours to process this package of fiber, in contrast to about 4 hours required for a spool oxidation cycle of the same amount of filaments by practicing the present invention.
The spool on which the carbon fiber is to be wound for oxidation obviously should not be damaged during the oxidation process and should not, in turn, damage the carbon fiber. More specifically, the spool must withstand the elevated temperatures of oxidation of the carbon fiber without undesirable sagging, collapse, expansion or other deformation. The elevated temperatures utilized for oxidation are generally in excess of 230.degree. C. and up to about 330.degree. C. Commercially available spools are of materials which expand when subjected to such oxidation temperatures and the diameters of the spools would increase. This will have a number of undesirable results. Firstly, when the spool expands it pulls the wound fiber tighter, which inhibits penetration of oxidizing gas through the bundle of fiber. Secondly, the fiber might be caused to fuse or break, if the spool expands inside the wound fiber. In addition, there is also the possibility that the fiber itself may suffer 1-2% shrinkage during oxidation, which would compound the problems caused by spool expansion.
U.S. Pat. No. 4,351,816 describes a spool for holding oxidized or thermoset carbon fibers during the much higher temperature carbonizing step, which follows the oxidation step here under discussion. The patent describes the bobbin there as being comprised of a carbon or graphite material. However, the material of the bobbin of this invention is not suggested, and there is no indication that the characteristics of the present bobbin were obtained.